It's really just ground-layering, but you have to decide where you want it to root. Some rafts are curvy and only root at certain intervals, while others root almost all the way across. You should really plan the final image before you try to root it.

The branch in particular has four branches coming off it, all on one side. Since four is an inauspicious number, I want to grow out the "trunks" on the raft to have gnarly, twisted branches coming off them. When the tree defoliates in winter, I want it to have a creepy, haunted forest vibe. The idea is that it is a penjing that represents death.

So, I think probably rooting the raft all along the bottom is the way to go.